1. Field of the Invention
The present invention relates to an improvement of an arc welder of the type in which an alternating current is rectified to a direct current, the direct current output is converted to a high-frequency alternating current by an inverter circuit based on the PWM control, the voltage of the high-frequency alternating current is converted by a transformer to an adequate voltage, and the alternating current is then rectified to a direct current by a rectifier circuit.
2. Description of the Related Art
A conventional MAG welding power source grouping as the constant voltage type will be described below with reference to FIG. 4. In FIG. 4, the numerical reference 40 denotes a DC reactor of a current superposing circuit, 41 denotes a DC reactor of an output main circuit, 42 denotes a wire, 43 denotes a pair of wire feed rollers, 44 denotes a conductivity tip, and 45 denotes an arc load. The output main circuit is formed so as to elongate from terminals of a secondary winding 56 to the midpoint thereof through a rectifier 6, the DC reactor 41, the conductivity tip 44, the wire 42, the arc load 45, and workpieces. The current superposing circuit is formed such as to elongate from terminals of the secondary winding 56 to the midpoint of the secondary winding 56 through capacitors 51, full-wave rectifying diodes 52, a DC reactor 40, the DC reactor 41, the conductivity tip 44, the wire 42, and the arc load 45. The capacitances of the capacitors 51 might be selected so that the current flowing can be made sufficient for maintaining an arc although the current flowing through the current superposing circuit is smaller than that of a usual welding current. Hereinafter, the operation will be described. When the wire extension of the tip is excessively blown away at the starting period of an arc, which ends up raising the arc voltage, for instance, the welding current becomes insufficient. However, as the result of a current supplied from the current superposing circuit, the arc is not interrupted. Although exemplarily described in the above is a MAG welding power source of the constant voltage type, the above configuration may be also applied to a pulsed arc welding power source. In this case, the current to be supplied from the superposing circuit maybe set to be slightly smaller than the base current. Having this configuration, it is possible to prevent the arc from being interrupted, even when the feed resistance of a wire, being subjected to a welding process, is changed so that the arc length is so increased and arc interruption occurs under the base current such as disclosed in line 20 of right column of page 3 to line 8 of left column of page 4, or FIG. 4 of JP-A-5-318128.
In a conventional arc welder, however, it can not be easily done with a single arc welder to conduct both short circuiting arc welding and pulsed arc welding there-into. This is because the welding current in short circuiting arc welding is required for being moderately changed, whereas the pulsed arc welding requires steep change. The DC reactor denoted by 41 in FIG. 4 controls the welding current change. In short circuiting arc welding, in general, a DC reactor of 30 to 200 μH (microhenries) is used, while in pulsed arc welding a DC reactor of 5 to 20 μH is used.
In case of both the short circuiting arc welding and pulsed arc welding being performed by a single welder, so-called an electronic reactor control method is used. This method uses a DC reactor of a small reactance for pulsed arc welding, however, the shortage in the reactance can be compensated by controlling a controlling circuit 17 of FIG. 4 so as to produce a current change resembling to an enhancement of the DC reactor. In other words, since reactance is difficult for being reduced, but it can be increased, the reactor in the hardware is structured so as to have a small reactance suitable for pulsed arc welding, while short circuiting arc welding can be performed with using a combination of the small reactance and the electronic reactor.
In MAG short circuiting arc welding using a large current or short circuiting arc welding under CO2 arc welding, however, a stable welding state is difficult by relying only on the combination control of the small reactance and the electronic reactor by said electronic reactor control method. In a related art, two types of welders, an arc welder for short circuiting arc welding having a large reactance, and another one for the pulsed arc welding having a small reactance shall be set at the same time. Therefore, its user must prepare the two types of welders, which causes the problems of increasing the apparatus cost, increasing the numbers of the spare parts, or requiring the time for exchanging two welders, depending on the welding method being used.
In the circuit of another related art, since the capacitors 51 are connected in series to the diodes 52, peak amount charge can be immediately performed. Therefore, given that the output voltage of the transformer is indicated by Vi, that of the superposing circuit by Vo, the current of the superposing circuit by Io, and the capacitance of each of the capacitors by C, the energy Vo∘Io output from the current superposing circuit is proportional to the charging/discharging energy C∘Vi∘Vi of the capacitor, or proportional to the square of the output voltage Vi of the transformer. In the PWM control, the energy is changed by controlling the ON time while the voltage Vi is maintained constant, and the value of C∘Vi∘Vi in the conventional art is therefore approximately constant. In other words, in the case where the output voltage Vo is low, such as the case of a short circuit, the current Io is abnormally increased. On the other hand, when the output voltage Vo is constant, the output current Io is generally constant irrespective of the value of the main current. As the result, if a large current is to be supplied to the superposing circuit when the main current is large, then a large current would be eventually supplied to the superposing circuit even in case of the main current being small. In this regard, it is impossible to perform both short circuiting arc welding and pulsed arc welding by a single welder.